1. Field of the Invention
The present invention relates to a method of inspecting a crystal defect or the like in a surface semiconductor layer of an SOI (silicon on insulator or semiconductor on insulator) substrate serving as a semiconductor substrate.
2. Description of the Background Art
An SOI substrate is generally manufactured by a bonding method or an SIMOX (separation by implanted oxygen) method.
In general, a surface silicon layer of an SOI substrate manufactured by the bonding method (hereinafter also referred to as "bonding SOI substrate") has defects such as voids (hollow defects) referred to as COPs (crystal originated particles) and an oxygen precipitate intrinsic to a bulk substrate employed as the raw material as well as an oxygen precipitate and stacking faults induced in steps of manufacturing the SOI substrate.
In an SOI substrate manufactured by the SIMOX method (hereinafter also referred to as "SIMOX substrate"), on the other hand, thin regions or surface depressions, for example, may be caused on the surface silicon layer in manufacturing steps in addition to COPs intrinsic to a silicon substrate employed as the raw material. When carrying out an oxygen implantation step on the surface silicon layer of the silicon substrate still having particles and the like, portions located under such particles and the like are reduced in thickness as compared with the remaining portions of the same layer since the depth of oxygen ions implanted into these portions is smaller than that in the remaining portions. Further, the amount of the oxygen ions implanted into the portions located under the particles and the like is also smaller than that in the remaining portions. When performing heat treatment after oxygen implantation for forming an embedded oxide (silicon oxide) layer, therefore, the amounts of expansion of the silicon oxide vary with the different amounts of oxygen implantation and the regions located under the particles into which oxygen is implanted in a smaller amount, i.e., in which the silicon oxide expands by a smaller amount, are depressed with respect to the peripheral regions.
In addition to the aforementioned inconveniences caused in the surface silicon layer itself in the initial stage, inconveniences such as an oxygen precipitate and stacking faults resulting from each manufacturing process are induced in the surface silicon layer after the SOI substrate is introduced into the manufacturing steps.
A hydrofluoric acid dipping method (HF dipping method) is widely employed as a method of inspecting such inconveniences of the surface silicon layer. The conventional inspection method is now described. FIG. 14A is a longitudinal sectional view of an SOI substrate 1 not yet subjected to hydrofluoric acid dipping. As shown in FIG. 14A, the SOI substrate 1 comprises a substrate support part 30, an embedded oxide layer 20 formed on the support part 30 and a surface silicon layer 10 formed on a surface 20S of the embedded oxide layer 20 opposite to the support part 30. FIGS. 14A and 14B typically illustrate COPs 51a to 51d (hereinafter also generically referred to as "COPs 51"), an oxygen precipitate 52 and a locally thin region 53 as defects 50 which are initial inconvenient portions of the silicon layer 10.
The SOI substrate 1 in the state shown in FIG. 14A is dipped in a hydrofluoric acid solution. Then, the hydrofluoric acid solution erodes the embedded oxide layer 20 through the COP 51a reaching the aforementioned surface 20S from an exposed surface 10S of the surface silicon layer 10, as shown in FIG. 14B. Thus, an embedded oxide layer 21 having an eroded part 21a under the COP 51a is defined. The density etc. of the COP 51a in the SOI substrate 1 can be recognized by observing the eroded part 21a. Thus, the hydrofluoric acid dipping method enables visual inspection of an SOI substrate with an optical microscope or the like by visualizing defects of sizes generally unobservable with an optical microscope.
The hydrofluoric acid dipping method, which visualizes the defect of the surface silicon layer 10 by eroding the embedded oxide layer 20 with hydrofluoric acid, is applicable to a part such as the COP 51a shown in FIG. 14A where the hydrofluoric acid solution can reach the embedded oxide layer 20, i.e., a part not formed with the surface silicon layer 10 but exposing the embedded oxide layer 20. In other words, only a defect reaching the surface 20S of the embedded oxide layer 20 from the exposed surface 10S of the surface silicon layer 10 can be detected with the hydrofluoric dipping method. More specifically, the hydrofluoric acid dipping method is inapplicable to inspection/evaluation of defects (hereinafter also generically referred to as "inner defects 5ON") such as (I) the COP 51b not reaching the surface 20S, (II) the COP 51c not reaching the surface 10S, (III) the COP 51d and the oxygen precipitate 52 reaching neither of the surfaces 10S and 20S and (IV) the thin region 53 shown in FIG. 14A.
When forming a semiconductor element or the like on the surface silicon layer 10 in general, the thickness of the surface silicon layer 10 is reduced into a level suitable for the semiconductor element before the same is introduced into the manufacturing steps. If the inner defects 50N can be inspected before reducing the thickness of the surface silicon layer 10, therefore, reduction of the manufacturing yield can be prevented beforehand. In the hydrofluoric acid dipping method which is a conventional inspection method, however, the inner defects 50N of the SOI substrate 1 cannot be sufficiently grasped in advance of introduction into the manufacturing steps. Even if the SOI substrate 1 is determined as non-defective through inspection by the hydrofluoric acid dipping method before introduction into the manufacturing steps, therefore, it is impossible to avoid such a situation that the inner defects 50N appear after reducing the thickness of the surface silicon layer 10 to remarkably lower the manufacturing yield.